1. Field of the Invention
The present invention relates generally to distributed, electronic process control systems having a number of remotely located stations interactively communicating in the control of a complex industrial process. More particularly, this invention relates to a method and apparatus for providing a secure, multimaster communication path among a plurality of remotely located stations of a process system.
2. Description of the Prior Art
The complexity of industrial processes being monitored and controlled has evolved from simple single control-loop processes to very complex processes often having hundreds of control loops. Industrial process control equipment has followed this evolution. From very simple analog controllers, today's process control equipment has developed into sophisticated electronic systems, often including digital computers.
One level of this sophistication is the use of distributed systems that call for individual stations to be remotely positioned throughout the process plant or centrally located, but separate from, the operator's control room. The stations are typically connected for communication with one another via a number of transmission cables stretching throughout the plant. As the process becomes more and more complex, the number of stations, and thus the cables, increases dramatically. This increase in cables greatly reduces system reliability, and increases system cost.
To reduce the number of cables, one prior art proposal teaches a polling technique in which all the stations are connected by a single cable, or by two cables if redundancy is desired.
A supervisory controller, typically a host computer, is centrally located and connected to all stations by the cable. The supervisory controller is the master of all communication and "polls" all stations according to their respective priorities to determine if the polled station wishes to communicate with another station.
If a station answers affirmatively, the supervisory controller acknowledges and waits until the communication is completed to restart its polling of the stations. If a station answers negatively, the supervisory controller polls the next station in line.
This technique is acceptable for use with processes having slowly varying process variables. It is not acceptable if the process variables change rapidly. For if the rate of change of a process variable is greater than the frequency at which its respective station is polled, variations in the variable may be missed, possibly causing a disruption or "bump" in the process under control. These bumps can cause severe safety problems, especially if an alarm limit indication is missed.
Two other factors negate the utility of this technique in a process control environment. First, the supervisory controller is a single-point-failure element. If the supervisory controller fails, all system communication, and thus process control, is disabled. Second, the stations, once allowed to communicate, may do so indefinitely, completely disabling communication by the other stations. This can also lead to missed indications, and more severely, to a complete loss of system communication due to the blocking effect of the then communicating stations. A loss of communication, which if produced, could completely disable all control of the process.